I. Field of the Invention
The present invention relates generally to mobile radio communication systems. More particularly, the present invention relates to power control systems for controlling the power level of signals sent on the forward links of a mobile radio communication system, and for controlling the signal to noise ratios of signals sent on the reverse links of a mobile radio communication system. Even more particularly, the present invention relates to a novel system and method that dynamically adjusts the parameters of forward and reverse link power control loops based upon the position and velocity of a mobile station.
II. Description of the Related Art
In mobile radio communication systems such as, for example, code division multiple access (CDMA) communication systems, the transmit power levels of signals sent between a base station and a mobile station are carefully controlled using power control loops. Typically, one power control loop is used to maintain the power level of signals on the forward link (i.e., signals sent from the base station to a mobile station) between a minimum and a maximum level. A different power control loop is similarly used to maintain the received signal to noise ratio of signals sent on the reverse link (i.e., the SNR measured at the base station of signals sent from a mobile station to the base station) between a minimum desired level and a maximum desired level. These power control loops typically adjust the transmit power of a signal upward or downward in fixed step wise increments (e.g., plus or minus 1 dB) in order to maintain either the power level or the signal to noise ratio of the signal between the minimum and maximum desired levels.
In early CDMA systems, forward link power control was typically accomplished using an open power control loop, whereas the reverse link power control system used a closed power control loop. Current CDMA systems use a closed power control loop for forward link power control and it is expected that future CDMA systems will continue to use a closed power control loop for forward link power control.
Each geographic cell in a mobile radio communication system is typically serviced by a base station associated with the cell. In current systems, the parameters of the forward link power control loop (i.e., the minimum power level, maximum power level, and step size associated with the forward link power control loop) and the limits of the reverse link power control loop (i.e., the minimum desired signal to noise ratio, maximum desired signal to noise ration, and step size associated with the reverse link power control loop) do not vary as the location and velocity of mobile units within the cell varies.
The transmit power requirements of signals sent between a base station and a mobile station vary as the mobile station moves within a cell. For example, as the mobile station moves closer to the base station, less transmit power is often required on the reverse link to achieve the same signal-to-noise ratio of the received signal at the base station. Thus, as the mobile station moves closer to the base station, the transmit power on the reverse link can often be reduced while maintaining the same signal-to-noise ratio of the received signal at the base station. In addition, less transmit power is also often required when the mobile station is stationary or moving slowly (as opposed to moving quickly) to achieve the same signal-to-noise ratio of the received signal at the base station. Thus, as the mobile station slows down, the transmit power on the reverse link can often be reduced while maintaining the same signal-to-noise ratio of the received signal at the base station.
The capacity of a given mobile radio communication system is limited by the transmit power available to the base station and the signal to noise ratio requirement on the reverse link. Thus, the capacity of the system could be increased if the base station transmit power and/or the signal to noise ratio on the reverse link in the system are reduced. In view of this, it would be desirable to have a power control system that could dynamically adjust the parameters of the forward and reverse link power control loops in response to the changing position and/or speed of a mobile station, so as to conserve transmit power and increase cell capacity.
The present invention maximizes the capacity of a given cell by adjusting one or both limits of a power control loop based on the position of a mobile station within the cell. In one embodiment, the present invention dynamically adjusts the parameters of a power control loop that attempts to maintain the transmit power of a signal sent from a base station to a mobile station above a minimum threshold. In this embodiment, location information that is representative of the distance and the morphology between the mobile station and the base station is determined. The minimum threshold of the power control loop is then adjusted in accordance with the location information such that the minimum threshold varies as the distance and the morphology between the mobile station and the base station change. A transmitter then transmits the signal at a power level that is controlled by the power control loop with the adjusted minimum threshold. This aspect of the invention can be used for controlling the minimum power level associated with the forward link power control loop.
In a further embodiment, the present invention dynamically adjusts the parameters of a power control loop that attempts to maintain the signal to noise ratio of a signal sent from the mobile station to the base station above a minimum threshold. In this embodiment, location information that is representative of the distance and the morphology between the mobile station and the base station is determined. The minimum threshold of the power control loop is then adjusted in accordance with the location and the morphology information such that the minimum threshold varies as the distance and the morphology between the mobile station and the base station change. A transmitter then transmits the signal at a power level that is controlled by the power control loop with the adjusted minimum threshold. This aspect of the invention can be used for controlling the minimum desired signal to noise ratio (i.e., the minimum desired signal to noise ratio of a reverse link signal received at the base station) in a reverse link power control loop.
In accordance with further alternative embodiments, the present invention also determines velocity information that is representative of the velocity of the mobile station. In these embodiments, the minimum threshold of a power control loop is adjusted based on the velocity information alone or, alternatively, the minimum threshold of the power control loop is adjusted based on both the velocity information and the current position of the mobile station. This aspect of the invention can also be used for controlling the minimum thresholds associated with both the forward and reverse link power control loops, and is especially useful in mixed wireless networks where there are both fixed position wireless users (also referred to as wireless local loop users) and mobile wireless users (i.e., wireless users that change position.) In such mixed wireless networks, adjustment of the minimum thresholds associated with both the forward and reverse link power control loops based on the velocity of wireless user ensures efficient allocation of system resources.
In accordance with a still further aspect, the maximum threshold of a forward link power control loop (i.e., the power level below which the power control loop attempts to maintain the transmit power of the signal sent from the base station) is also adjusted in accordance with the location information (and optionally the velocity information) such that the maximum threshold varies as the distance and the morphology between the mobile station and the base station (and optionally the velocity of the mobile station) vary.
In accordance with yet a further aspect, the maximum threshold of a reverse link power control loop (i.e., the maximum desired signal to noise ratio below which the power control loop attempts to maintain the signal to noise ratio of a reverse link signal received at a base station) is also adjusted in accordance with the location information (and optionally the velocity information) such that the maximum threshold varies as the distance and the morphology between the mobile station and the base station (and optionally the velocity of the mobile station) vary.
In accordance with a still further aspect, the step size of a power control loop (i.e., the step wise power increment used in the power control loop to adjust the transmit power of the signal upward or downward) is also adjusted in accordance with the velocity information such that the step size varies as the velocity of the mobile station varies. In accordance with this aspect, the step size used by the power control loop is preferably reduced for stationary or slow moving mobile stations, and the step size is increased when a mobile station begins moving more quickly. This aspect of the invention can be used for controlling the step size of either a forward link or reverse link power control loop.